This invention relates generally to an electrical power control system for driving a motor, and more particularly relates to a voltage switching arrangement for connecting and disconnecting voltage from the input to a power transfer network.
Many AC motor control systems used heretofore included mechanical power line switches for connecting and dis-connecting voltage from the input to a solid state power switching or transfer network. The electrical power may be transferred from the input to the output of the power transfer network for driving the motor in the forward and reverse directions. When turning the motor system off, the mechanical switches would disconnect from the input to the power transfer network before the power transfer network was switched from an on-mode to an off-mode. This caused electrical arcing which could permanently damage or substantially reduce the life of components used in the motor control. The damaging effect of this arcing is reduced or suppressed if capacitors and resistors are connected in shunt across the static switches of the transfer network. When large currents are transferred to the motor, such suppressor capacitors are very costly and are large in size, and may not be effective for suppressing all the sharp and damaging spikes on the current power lines.
U.S. Pat. No. 3,716,770 (1973), VICTOR J. HABISOHN, disclosed an AC motor control system for driving the motor in the forward or reverse direction and included a contactor "break" arc suppressor means. When either the forward or reverse manual push button switch was released, the AC energizing voltage was removed from the solenoid coil, causing a disable signal to be generated for switching the solid state power switching or transfer network from the on-mode to the off-mode. The power line mechanical contactor switches opened or disconnected from the input to the power transfer network after the power transfer network switched to the off-mode. This was due to the relatively slow reaction of the mechanical contactors as compared with the quick reaction of the transfer network. Hence, electrical arcing was prevented from occurring on the contactor break.
However, the VICTOR J. HABISOHN technique for preventing arcing on the break of the contactor switches is not suitable for the rapid switching mechanical relays and switches that are presently being used in motor control systems for switching electrical power. These relays switch the power line switches from the on to the off condition almost instantaneously after the relay energizing voltage is removed, and prior to the static switches in the power transfer network switching from the on to the off-mode. Thus, damaging electric arcs are generated across the relay power line switches. The invention herein prevents electrical arcing of the mechanical power line switches when switching from an on to an off-condition.
Another problem with prior motor control systems was the possibility of switching the power line voltages to the input to the power transfer network simultaneously in the forward direction and in the reverse direction, and causing severe damage to the motor or control system. This could have occurred upon inadvertently closing both the forward and reverse manual push button switches, which determine the rotational direction of the motor. The subject invention includes safeguards to prevent the simultaneous connection of the voltage to the input of the power transfer network, in both the forward and reverse directions.